Multiple layer sheet material

ABSTRACT

The invention is an improved multiple layer packaging sheet material which can be formed into closed and sealed packages suitable for holding products containing substantial fractions of cyanoacrylate. The invention depends on the positioning of high density polyethylene in direct contact with the cyanoacrylate-type product, and positioning a primer on the other side of the high density polyethylene. Preferred embodiments of the sheet structure include additional layers to provide barrier and abuse resistance properties, and the necessary adhesive materials to hold the various layers of the structure together with good adhesion. Additional layers are contemplated. The sheet structure can be formed into a package which positions the high density polyethylene adjacent the cyanoacrylate product, and between it and the other layers at all locations of contact between the cyanoacrylate product and the packaging sheet material.

This is a continuation, of application Ser. No. 687,202, filed Dec. 28,1984 pending.

BACKGROUND OF THE INVENTION

Products containing cyanoacrylate are quite active chemically. Becauseof the reactive nature of cyanoacrylates, it has been difficult to findpackaging means and materials with which the cyanoacrylate product doesnot interact in a way which is detrimental to the cyanoacrylate in thepackage, and to holding it in the package.

Traditionally, cyanoacrylate adhesives have been packaged in plasticbottles and metal tubes. The plastic containers do not provide adequatevapor barrier, such that the shelf life of products packaged inconventional plastic containers is undesirably short. Metal tubes alsohave some undesirable characteristics. They are relatively inflexiblesuch that they can be flexed only a few times as in repeated dispensingbefore cracks develop in the tube sidewall. Overall, though, shelf lifeof cyanoacrylate products is better in metal tubes than in plastic, atleast until the tube is initially opened. Nevertheless, the shelf life,even in metal tubes, is less than desired. Clearly, cyanoacrylates arereadily identified as being difficult to hold in a package over adesirable shelf life period.

Products have been packaged in tube-type containers of generally threetypes. The first type is a metal tube. Such tubes provide excellentbarrier properties, but have undesirable functional properties asdiscussed above. The second type is a plastic tube made from eithersingle or multiple layers of plastics. While plastic tubes may beexcellent for many products, no plastic packaging materials are knownwhich are excellent for holding cyanoacrylate-type products. The thirdtype is a tube made from previously formed multiple layer sheetmaterials containing a layer of metal foil and a plurality of layers ofplastic materials, generally referred to hereinafter as laminated tubes.

The third type of tube container has been used conventionally forholding hygienic-type products such as toothpaste. Toothpaste ischemically quite active, and is generally considered difficult to holdin a package.

Attempts to package cyanoacrylate adhesives in the third type of tubecontainers were not successful with any of the conventionally knownsheet materials used to make the laminated tubes. After being packagedin conventional tubes of the third type, the cyanoacrylate adhesivemigrated through polymer layers to the foil layer. The tube delaminatedbetween the foil layer and the adjacent polymer layer. The delaminatedtube was not satisfactory for the package.

Cyanoacrylate also tends to cause swelling of the polymers. In trialswith some experimental multiple layer tubes, polymer layers disappearedas distinct layers, presumably having been either dissolved into thecyanoacrylate or otherwise absorbed into it.

It is an object of this invention to provide a packaging sheet materialcapable of holding cyanoacrylate-type products.

It is another object of this invention to provide such a sheet materialwhich has excellent barrier to passage of matter through the sheetmaterial.

It is yet another object to provide a package capable of holding acyanoacrylate-type product.

SUMMARY OF THE INVENTION

These and other objects are achieved in a multiple layer sheet materialhaving all the layers firmly adhered to each other. The sheet materialhas a first layer of high density polyethylene (HDPE), a second layerwhich may be a polymer or an adhesive, and a third primer layer ofpolyethylene imine (PEI). The third primer layer is between the firstand second layers.

Regarding the layer compositions, each of the layers has preferredfeatures which, while not critical to the invention, contribute to itsefficiency or optimization. Thus, the resin density of the HDPE asdetermined by ASTM D-2839 is preferably at least 0.950. It should alsopreferably have a narrow molecular weight distribution and should besubstantially free of amine components. Preferably its crystallinity isrelatively high as developed in formation of that film-like layer.

The second layer may be any of a wide variety of functional materialswhich exhibit the desired properties. Preferred for the second layer isethylene acrylic acid copolymer (EAA).

The primer in the third layer is preferably applied as a liquid solutioncontaining a cross-linking type PEI.

Preferred embodiments of the sheet material include a fourth layer ofmetal foil so positioned in the structure that the second layer isbetween the third and fourth layers.

Finally, in most preferred embodiments, a fifth layer adheres a sixthlayer, as of polyethylene, to the metal foil.

Another aspect of the invention is seen in a package which includes thenovel packaging sheet material herein in the form of a package and aquantity of product therein where the product contains a substantialfraction of a cyanoacrylate-type component. The package is configuredsuch that no edge of the sheet material is exposed to the product.

Still another aspect of the invention is seen in a package wherein thepackaging sheet material has been modified by virtue of contact withcyanoacrylate-type components. In the modified form of the sheetmaterial, the primer layer includes reacted components from thecyanoacrylate products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a three-layer sheet structure of thisinvention.

FIG. 2 is a cross-section of a four-layer sheet structure of thisinvention.

FIG. 3 is a cross-section of a six-layer sheet structure of thisinvention.

FIG. 3A is a cross-section of sheet structure not of this invention. Itis similar to sheet structure of FIG. 3, except without the primer layerof FIG. 3.

FIG. 3B is a cross-section of another sheet structure not of thisinvention. It is similar to sheet structure of FIG. 3 but has the primerlayer in the wrong location.

FIG. 3C is a cross-section of a sheet structure of the invention whichhas two primer layers.

FIG. 4 is a pictorial view of an exemplary package of this invention,and including a cyanoacrylate-type product therein.

FIG. 5 is a greatly enlarged cross-section taken at 5--5 of FIG. 4, andshowing the primer layers immediately after filling the package withproduct.

FIG. 6 is a greatly enlarged cross-section as in FIG. 5 shown after along enough time that the primer layer has been modified and includesreacted components from the product.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In FIG. 1 is shown a cross-section of a simple sheet structure 110 ofthe invention, including two polymeric layers 112 and 116 and a thinpolymer layer 14 therebetween. Layer 112 is HDPE. The composition oflayer 116 is usually polymeric and may be chosen with substantialfreedom. Layer 114 is a thin layer of primer.

In the numbering system, the first digit represents the FIGURE number.The second and third digits represent the element in the FIGURE. Commonsecond and third digit numbers in the various FIGURES represent commonelements. Thus 14 represents the primer layer in any of the FIGURESwhere it is shown. 114 represent the primer layer in only FIG. 1.

FIG. 2 shows a cross-section of a sheet structure 210 of the inventionof intermediate complexity. Layer 212 is HDPE. Layer 214 is a thinprimer layer. Layer 216 is an adhesive which is effective to adhere to afoil layer 218. With the introduction of the foil layer into thestructure, the composition of adhesive layer 216 is selected such thatit provides good adhesion between the foil layer 218 and the HDPE layer212 through the primer 214. EAA is preferred for layer 216.

FIG. 3 shows a cross-section of a sheet structure 310 of the inventionhaving more complexity than the structures of FIGS. 1 and 2, and alsohaving more-preferred functional properties. Layers 312, 314, 316, and318 are equivalent to the respective ones of those layers in FIG. 2.Layer 320 is an adhesive layer which adheres layer 322 to the foil layer318. Layer 322 is protective of the foil layer and its composition isselected with that protection property as an important criteria.

As stated earlier, cyanoacrylate is highly reactive as a chemicalproduct. It is this strong tendency toward reaction that makes it sodifficult to design a packaging material to hold it. It is furtherimportant that certain layers of the packaging material of thisinvention not be permitted to come into direct contact with thecyanoacrylate product. As it has been found that direct contact withcyanoacrylate can be tolerated by HDPE, it is preferred, and highlyfunctional for the cyanoacrylate-type product to be in direct contactwith only the HDPE layer in the finished packages of this invention. Anacceptable package structure is a fin-sealed pouch 411 as seen in FIG.4. The peripheral seals 424 are pointed out on 2 of the 4 sides of thepouch.

FIG. 5 shows a cross-section of the pouch of FIG. 4. Each of the layers12, 14, 16, 18, 20, and 22 is shown in its overall relationship to thesheet structure, the package, and the product. It is seen that thefin-sealed type of construction ensures that the cyanoacrylate producthas direct contact only with the HDPE layer. The FIG. 5 structure isaccurately descriptive of the package at the time the product is putinto the package.

As is illustrated in FIGS. 1, 2, 3, and 5, the primer layer 14 isextremely thin. It is important that the primer layer be continuous, inorder to meet its minimum required functional parameter. Meeting theminimum required functional parameters, however, does not impose anyminimum requirements for physical thickness of the primer layer beyondthat required for continuity. The typical quantity of a PEI present inthe sheet structure is of the order of 0.04 lbs. per 3000 square footream. In practice, the presence of layer 14 as a distinct layer is notvisually detectable by normal microscopic examination of a cross-sectionof the sheet structure up to the time it is brought into contact with acyanoacrylate-type product.

FIG. 6 represents the cross-section as in FIG. 5, but after thecyanoacrylate product has been in the package for a while. Thesignificant change is that a visible layer 614R has developed at layer14, which was also the interface of layers 12 and 16 for appearancepurposes when the sheet material was formed. For example, in packagesevaluated after one week of storage at 120° F., the new layer as at 614Rwas 0.4 to 0.7 mil thick. Packages evaluated after four weeks of storageat 120° F. had layers 614R which were 0.9 to 1.6 mils thick. A primarycomponent of the new layer 614R is reacted polymer of cyanoacrylate.Notably, the storage conditions cited herein are unusual in that theyaccelerate, in many respects, the normal process of aging for both thepackaging material and the product. Thus, the tests iterated herein mayrepresent the entire normal shelf life of the packaged product, or evenbeyond.

In some experimentally-made sheet materials which were otherwise thesame as the sheet materials 10 of the invention, the primer layer 14 wasnot continuous, but unintentionally had small discontinuities ofpin-hole size. Packages made with these sheet materials were notsatisfactory. On inspection of the filled and sealed packages after ashort storage period of 1 or 2 weeks, spot delaminations wereunexpectedly observed. On detailed inspection, discontinuities wereidentified adjacent each spot delamination. The discontinuities werevisible to microscopic inspection, in the reacted layer 614R. No spotdelaminations were observed which could not be associated with adiscontinuity. Thus the inventors have reached the conclusion that theprimer layer 14 serves some function of the nature of impeding themigration of the cyanoacrylate product to the surface of the foil 18. Itappears that the cyanoacrylate does penetrate, and pass through anydiscontinuity of the primer layer 14, to swell the layer 16, and reactat the foil layer 18 to cause delamination. The inventors herein haveconcluded that the continuity of primer layer 14 is critical tooperability of the invention. To that end, it is desirable to treat thesurface of the HDPE layer, as necessary, such as by corona discharge, toensure good wetting of the HDPE layer by the PEI when it is applied.

In certain cases, it may be desirable to have additional layers betweenHDPE layer 12 and the primer 14. Such additional layers may be used solong as any interaction they may have with, or as a result of, theproduct is acceptable and so long as the primer layer is continuous asdescribed earlier and is separated from any foil layer by at least oneintervening layer.

Experimentation has shown that, of the polyethylene imines available,the cross linking types are preferable over the non-crosslinking types.It is hypothesized that the PEI may act as a chemically basic electrondonor material, and may interact with the product in polymerization, orother reaction of the cyanoacrylate product. Once the cyanoacrylate hasreacted, the size of the reacted molecule is apparently such that it isno longer as mobile in the laminated sheet structure, and remains in thevicinity of the reaction site to form a part of the visible layer 14R.Based on the belief that the PEI acts as a catalytic electron donor, itis believed that other Lewis bases, such as amines and salts of weakacids, should function in a similar manner to impede migration of theproduct.

Essentially, the presence of the primer layer 14 impedes migration ofthe cyanoacrylate through the structure past the primer layer. FIG. 3Arepresents a cross-section of sheet structure similar to the six layerstructure of FIG. 3, but without the primer layer 14. Layers 312A, 316A,318A, 320A and 322A all represent layers equivalent to the respectivelayers in the sheet structures of the invention. Without a primer layer14, though, the sheet structure is not capable of satisfactorily holdingcyanoacrylate-type products without significant deleterious affect onthe packaging sheet structure. Typical affects are swelling of polymerlayers and delamination. FIG. 3B represents a cross-section of sheetstructure similar to the structure of FIG. 3, but with the primer layer14 in the wrong place. Layers 312B, 314B, 316B, 318B, 320B, and 322B allrepresent layers equivalent to the respective layers in the sheetstructure of the invention. Primer layer 14, however, is at theinterface between the foil 18 and the EAA 16, rather than at theinterface between the HDPE 12 and the EAA 16. With the primer thusimproperly positioned, namely adjacent the foil rather than displacedfrom it, the sheet structure is not capable of satisfactorily holdingcyanoacrylate-type products. Typical affects are swelling, especiallylayer 16, and delamination, particularly at the foil interface. Primersmay be used elsewhere in the invention, as seen in FIG. 3C, so long asthe primary layer of primer between layers 12 and 16 is retained. Tothat end layer 312C, 314Cl, 316C, 318C, 320C, and 322C all representlayers equivalent to the respective layers in FIG. 3. Layer 314C2 is anadditional primer layer which may, for example, be used to promoteadhesion between layers 320C and 322C. This practice of using one ormore additional primer layers, in addition to the primary primer layerat the interface of layers 12 and 16, is fully satisfactory so long asprimary primer layer 14, as described herein, is displaced from theproduct and from the foil by at least one layer, and is between theproduct and the foil.

In general, of the polymers usually associated with good heat sealingcapabilities, HDPE is the least affected by cyanoacrylate-type product.Other polymers are caused, by cyanoacrylatetype products, to swell by50% to over 150%. As in the case of linear low density polyethylenecopolymer (LLDPE), some polymeric materials are even absorbed into theproduct. While HDPE is also swelled somewhat, in the better HDPE's, theswelling is limited to only about 30%, and overall strength of thepackage is sustained satisfactorily. While most high densitypolyethylenes are acceptable, some are more preferred. The preferredpolymers are characterized by the following properties. Resin densityshould be at least 0.950. The HDPE preferably has a narrow molecularweight distribution. A high degree of crystallinity in the HDPE layer isdesirable, and can be somewhat controlled by the rate of cooling whenthe HDPE layer is formed by some melt forming process such as extrusion.

The critical elements of the invention are the HDPE layer 12 whichserves as the sealant layer without being unacceptably affected by thecyanoacrylate product, and the primer layer 14. The presence of theprimer layer at the disclosed location causes the mobility of thecyanoacrylate product to be impeded such that it does not readily passthrough layer 14 and have harmful affect on the layers 16, 18, etc.,which are beyond layers 12 and 14, as viewed from the layer 12 surfacethrough the sheet material. Thus, the compositions of the various layers16, 18, etc., beyond layer 14 may not need to be selected for theirresistance to cyanoacrylate. Rather, they may be selected for theirother desirable properties. Thus, layer 16 is defined rather broadly, asa polymer or an adhesive, and may be selected for the propertiesdesired. In a three layer structure as in FIG. 1, layer 16 may, forexample, conveniently be any polymer which adheres well to the primedside of the HDPE layer. Typical layer 16 polymers would be low densitypolyethylene (LDPE), HDPE, and propylene ethylene copolymer (PPE). Wherea fourth layer is present in the sheet material, then the layer 16 mustadhere well to it as well as to the interface at layers 12 and 14. Inthese structures, layer 16 may be considered to be a layer servingprimarily an adhesive function. The adhesive layer may be a polymericadhesive. Where the fourth layer is metal foil, and an adhesive polymersuch as EAA is known to adhere well to foil and is preferred for layer16. Where the fourth layer 18 is polymeric, an adhesive polymer withgraft substitutions of carboxy moieties such as maleic anhydride may bepreferred. In some cases, adhesive emulsions may be used. In general,the selection of material for layer 16 depends on the definition of itsfunction in the sheet structure, whether it be of a protective nature,as may be true in a three layer structure, or of an adhesive nature instructures having more than three layers.

The compositions, then, of layers 12 and 14 are selected with theprimary purpose--in addition to their normal package structuralfunctions--of controlling migration of the cyanoacrylate product throughthe sheet material. When properly selected and applied, theyfunctionally control the migration of the cyanoacrylate through thesheet material, such that the compositions of all the other layers maybe selected without primary consideration of their susceptibility tocyanoacrylate. Thus layer 18 is a metal foil layer which would be highlysusceptible to loss of adhesion to polymers in the presence ofcyanoacrylate-type products. Rather it is selected for its excellentproperties as a barrier against passage of any matter through thepackage sheet material, into or out of the package. Other barriermaterials may be used in the invention. Along with selection of thebarrier material, if one is used, proper materials are selected fromthose known in the art, for the layers adjacent the barrier layer toensure adhesion to, and compatibility with, the rest of the sheetstructure. Exemplary of possible alternate barrier materials arevinylidene chloride copolymers, ethylene vinyl alcohol copolymers, andblends including major fractions of these.

Layer 16 is preferably EAA, another material susceptible of beinggreatly swelled by cyanoacrylate. But, with the protection provided bylayers 12 and 14, the EAA is not thus disastrously affected by theproduct and may be selected for its excellent adhesive properties to themetal foil. Likewise layers 20 and 22 may also be selected withoutprimary consideration of direct interaction with the product. By thesame consideration, the layers 16 through 22, as well as additionallayers may be selected with substantial freedom as long as the layers 12and 14 serve a protective function between the layers 16 through 22 andthe product.

The most preferred structure of the sheet material herein is thatillustrated by FIGS. 3, 5, and 6. The sealant layer 12 and the primerlayer 14 are narrow molecular weight HDPE and crosslinking PEIrespectively. "Narrow molecular weight" is a relative term, used by theindustry, that identifies the subject group of HDPE polymers bysubjectively comparing their molecular weight distributions with thoseof other HDPE polymers. Layer 16 is EAA which serves as an adhesive tothe metal foil barrier layer 18. Layer 20 is preferably EAA which servesas an adhesive between the foil layer 18 and layer 22 which is desirablya layer of polymer which is selected for either its excellent abuseresistance or its heat resistance. While a preferred composition forlayer 22 is HDPE, other materials may be preferred for some uses.Exemplary of these are oriented polypropylene, oriented polyester, andoriented nylon. In some preferred embodiments, the layer 22 compositionis LDPE. The composition of the adhesive layer 20 is selected in view ofthe selected compositions of both layers 18 and 22. Additional layersmay be used as dictated by the situation, such as for printing,pigmenting, etc.

In order for the combined properties of the HDPE in layer 12 and theprimer in layer 14 to provide protection for the other material layersin the sheet structure, namely protection from the cyanoacrylateproduct, it is important that the HDPE and PEI layers be interposedbetween the cyanoacrylate and the other layers at all areas where thecyanoacrylate is in contact with the packaging sheet material.Illustrative of this principle is a fin-sealed package as illustrated inFIGS. 4, 5, and 6. Formation of a finished package as shown in FIG. 4,from multiple layer sheet materials, is conventionally known in theindustry. While the sheet structures disclosed herein are highlyunusual, forming them into a finished package is relatively straightforward, as by means of conventionally forming heat seals about theperiphery as shown in FIG. 4. The development of the visible layer 614Ris, of course, highly novel and unexpected, and especially in its rolein impeding penetration of cyanoacrylate into the rest of the packagingstructure.

The process of making the sheet structure of the invention will now bediscussed in terms of the preferred embodiments of the more complexstructure; and it will be seen that the simpler structures are in somecases substructures thereof. The discussion of complex structures as inFIG. 3, thus, also includes the general types of processes which areused to make the simpler 3 or 4 layer structures.

In the first operation, a layer of HDPE is formed, as by an extrusionprocess. The process is designed for relatively slow cooling of theextruded HDPE layer in order to encourage development of crystallinity.A preferred process is tubular blown extrusion. Another acceptableprocess is cast extrusion where the sheet is cast onto a relatively warmroll and where the elevated temperatures are held over a period of time,down stream of the extrusion, which is conducive to the development ofcrystallinity. The HDPE layer is then preferably corona treated to alevel of at least 38 dynes/cm. of surface energy. The treated side ofthe HDPE is then primed with a PEI primer solution and dried, leaving acontinuous layer of PEI on the surface of the HDPE. The primed side ofthe HDPE layer is then extrusion laminated to aluminum foil using EAA asthe extrusion laminant. The structure at this state is represented byFIG. 2. The structure as represented by FIG. 3 is completed by extrusionlaminating a previously formed abuse resistant layer, such as HDPE, tothe exposed side of the foil, using EAA as the extrusion laminant.

If desired, the sheet material may then be reacted with cyanoacrylatemoieties, to form a sheet material having a reacted primer layer 14A,before forming it into a package structure as in FIG. 4. The reactionmay be carried out by contacting the HDPE layer 12 with thecyanoacrylate moieties.

In making the 3-layer structure, represented by FIG. 1, the HDPE layeris formed, treated and primed as previously discussed. Layer 16 is thenadded by an appropriate process. A typical process is extrusion coatingof the appropriate material onto the primed surface. In anotherexemplary process, layer 16 is formed in a separate operation and isjoined to primer layer 14 by heat and pressure lamination at a hot nip,or may be extrusion laminated to layer 14 by using an intervening layerof an extrusion laminant.

Thus it is seen that the invention provides a packaging sheet materialcapable of holding cyanoacrylate-type products. It further provides asheet material which has excellent barrier properties to passage ofmatter through the sheet material. Even more significantly, theinvention provides a sheet material which can be made into a closed andsealed package capable of holding a cyanoacrylate-type product.

The sheet materials of this invention are also, of course, capable ofholding products which have lower chemical activity than cyanoacrylate.In those cases, materials which are less preferred for holdingcyanoacrylate may be more desirable because of other advantages such aseconomics or processing advantages.

Having thus described the invention, what is claimed is:
 1. A containermade from a multiple layer sheet material, said sheet materialcomprising:(a) a first layer of high density polyethylene as an exteriorlayer of said sheet material; (b) a second layer, the composition ofsaid second layer being chosen from the group consisting of polymers andadhesives; and (c) a third primer layer of polyethylene imine,said thirdprimer layer being between said first and second layers and in contactwith said first layer, said first layer being disposed toward the insideof said container, all the layers of said sheet material being firmlyadhered to each other.
 2. A container as in claim 1 wherein said firstlayer is free of amine components.
 3. A container as in claim 1 whereinsaid third layer is derived from a liquid solution containingpolyethylene imine of the cross-linking type.
 4. A container as in claim1 wherein said first layer has a narrow molecular weight distribution,is substantially free of amine components, and has a relatively highdegree of crystallinity.
 5. A container as in claim 3 wherein said firstlayer has a narrow molecular weight distribution, is substantially freeof amine components, and has a relatively high degree of crystallinity.6. A container made from a multiple layer sheet material, said sheetmaterial comprising:(a) a first layer of high density polyethylene as anexterior layer of said sheet material; (b) a second layer, thecomposition of said second layer being chosen from the group consistingof polymers and adhesives; and (c) a third primer layer, the compositionof said primer layer comprising a major fraction of Lewis base,saidthird primer layer being between said first and second layers and incontact with said first layer, said first layer being disposed towardthe inside of said container, all the layers of said sheet materialbeing firmly adhered to each other.
 7. A container made from a multiplelayer sheet material, said sheet material comprising:(a) a first layerof high density polyethylene; (b) a second layer of an adhesive polymer;(c) a third primer layer, the composition of said primer layercomprising a major fraction of Lewis base, said third layer beingbetween said first and second layers, said first layer being disposedtoward the inside of said container; (d) a fourth layer of metal foil;and (e) a fifth layer adhering a sixth heat sealable layer to saidfourth layer,said second layer being between said third and fourthlayers.
 8. A container as in claim 7 wherein the composition of saidsixth layer comprises polyethylene.